The portion of carbon dioxide (CO2) in exhaled air is an important variable that can be measured as an alternative or in addition to electrocardiography or measurement of the oxygen saturation of the blood, both for diagnostic purposes and for continuous monitoring over a protracted period in the intensive care unit of a hospital or in the sleep laboratory. The processes disclosed in EP 0 309 666 A1, DE 39 36 825 C2, or U.S. Pat. No. 5,159,934 disclose processes for determining of the CO2 content of respiratory air by measuring the absorption of infrared radiation.
In the so-called “mainstream process”, the detector that measures the absorption of the infrared radiation is mounted directly in the airway of the patient. This presents the disadvantage that the result of measurement may give false readings by precipitation of moisture onto the optical elements. In the so-called “sidestream process”, a small part of the exhaled air is withdrawn and subjected to spectroscopic measurement by a device that is remote from the patient. Aside from the danger of possible obstruction of the suction hose, this process presents the disadvantage that slight variations in the CO2 concentration, which may contain information of importance for diagnosis, generally cannot be detected in practice by metrological means.
EP 0 653 919 B1 and CH 669 463 A5 disclose a process and/or a device for measurement of flow rate, volume of flow, temperature, and mean molecular weights of gases and mixtures of gases. To the extent that measurement of molecular weights is used for the determination of various exhalation parameters in pulmonary function diagnosis, these documents teach the use a separate sensor for determination of the carbon dioxide or oxygen concentration.